1. Field of the Invention
The present invention relates to a nonvolatile semiconductor storage device which writes data in a nonvolatile manner by applying a voltage to a variable resistor and a data writing method therefore.
2. Description of the Related Art
Recently, a nonvolatile memory having memory cells each containing a variable resistor and disposed in array with connecting at intersections between word lines and bit lines has attracted attention.
A phase-change random access memory (PCRAM) having a chalcogenide device as the variable resistor, a resistance random access memory (ReRAM) having a transition metal oxide element, a memory having a cell which changes a resistance value by forming a contacting bridge between electrodes by depositing metallic cation or destroying the contacting bridge by ionizing the deposited metal (CBRAM) and the like are known as such the nonvolatile memory. These variable resistance memories are characterizes by storing information by utilizing a change of the resistance value.
In the PCRAM, the chalcogenide device are controlled a process from heating to cooling according to the shape of current/voltage pulse, i.e. magnitude and width, applied to the chalcogenide device to change the phase into the crystal state or non-crystal state, thereby the resistance value of the device can be controlled (see Japanese Patent Application National Publication No. 2002-541613). The ReRAM is classified into bi-polar type and uni-polar type. The bi-polar type memory controls the resistance value of the cell according to a polarity of a current/voltage pulse applied to the transition metal oxide element. On the other hand, the uni-polar type memory controls the resistance value of the cell according to the magnitude, width and the like of a current/voltage pulse applied to the transition metal oxide element.
In the case of the uni-polar type ReRAM, the variable resistance memory writes data by applying a program voltage (for example, approximately 4.5V, the current value is approximately 10 nA) for 10 ns to 100 ns. As a result, the variable resistor is changed from a high resistance state to a low resistance state. This state change is called “program” or “set”. If an erase voltage of approximately 0.7V is applied to the variable resistor in which data is programmed so as to feed a current of 1 μA to 10 μA for 200 ns to 1 μs, the variable resistor is changed from the low resistance state to the high resistance state. This state change is called “erase” or “reset”.
In such a set operation or reset operation, a necessary program voltage or erase voltage is applied to the variable resistor only connected to a selected word line and bit line. On the other hand, non-selected word lines or bit lines needs to be loaded with a control voltage which acts as a reverse bias in order to prevent the diode from turning ON. However, the non-selected word lines or bit lines take much rise-up time because the number of the non-selected word lines or bit lines is larger than the number of the selected word lines or bit lines thus the total capacity of non-selected lines is larger than the selected lines. Therefore, if the non-selected word lines or the bit lines are turned ON/OFF at each data set timing, data write time increases.